Aiming for a differentiation from products of other companies or an increase in the visibility of a product, containers, such as bottles or PET bottles, which are filled with drinks, such as juice, to be sold, are often fitted with a printed heat-shrinkable label on the side of the containers. In so doing, materials, such as from the polystyrene series, the polyester series, the polyvinyl chloride series and the polyolefin series, are used as the material quality for the heat-shrinkable labels used.
In recent years, in order to increase productivity, obtaining an external appearance with a good shrink finish at a low temperature and in a short time in the process of wrapping the label around the bottle, is a requirement for heat-shrinkable labels fitted onto PET bottles and the like. At the same time, the property of a small natural shrinkage ratio is also becoming a requirement. This natural shrinkage is a phenomenon whereby shrinkage occurs gradually inside a warehouse storage at room temperature or in the summer, and if the natural shrinkage ratio is large, sometimes, the film dimensions vary, and, in a rolled state, problems by deformation and the like occur at the time of packaging or at the time of secondary processing.
Meanwhile, recently, from the viewpoints of resource-saving and environmental contamination prevention, along with PET flakes and pellets being increasingly regenerated from PET bottles recovered after use, it is becoming desirable that the specific gravity of the heat-shrinkable label is less than 1.00. The reason is, in the regeneration process of PET bottles, a specific gravity separator and an air separator are used for eliminating heat-shrinkable label grounds from sorted and ground PET bottles, and from the point of view of increasing the processing capability per unit of time, heat-shrinkable labels with a specific gravity of less than 1.00, which can be sorted and eliminated with a specific gravity separator, are desired.
Examples of such heat-shrinkable films with a specific gravity of less than 1.00 include stretched films from polyolefin series resin, such as polyethylene series resin and polypropylene series resin. However, these films have problems, such as insufficient film firmness (stiffness at ordinary temperature) and shrinking properties, and bad shrink finishability due to large natural shrinkage ratio.
As a technique for improving the above problems, for instance, a heat-shrinkable polyolefin series film having a surface layer containing a cyclic olefin series resin and a intermediate layer containing a resin having a polyethylene series copolymer resin as the main component has been disclosed (see for instance Japanese Patent Application Laid-open No. 2001-315260).
However, although this film addresses the problem of lowering the specific gravity, the firmness (stiffness at ordinary temperature) as an entire film is low, in addition, as the cyclic olefin series resin of the surface layer has poor resistance to oil. The film therefore has problems, such as, when an individual touches the surface of the film by hand, oil fat constituents from fingerprints and the like become attached easily, and if heat shrinking is carried out in this state, a phenomenon occurs, giving rise to bleaching and fine cracks (hereinafter referred to as “fingerprint bleaching”) in this portion with attached oil fat, decreasing the product value.
In addition a heat-shrinkable polyolefin series film having a surface layer containing, as the main component, a resin in which 60 parts by mass or more but no greater than 150 parts by mass of a linear low density polyethylene resin is mixed with respect to 100 parts by mass of a cyclic olefin series resin, and a intermediate layer containing a propylene-α-olefin random copolymer as the main component has been disclosed (see for instance Japanese Patent Application Laid-open No. 2002-234115).
With the above film, although resistance to the fingerprint bleaching has been improved, compatibility between the cyclic olefin series resin and/or the linear low density polyethylene resin of the surface layer and the propylene-α-olefin random copolymer of the intermediate layer is poor. The film therefore has problems such as, when a recycled resin generated from trimming losses and the like, such as the edges of a film, is added (hereinafter referred to as “regeneration additivity”), sometimes the transparency of the entirety of the film decreases easily, or the interlayer adhesive strength between the surface layer and the intermediate layer is insufficient.
In addition, as the stiffness of polypropylene is generally high, if laminated, there is a tendency to decrease the shrinking properties of the film by preventing the shrinkage of the surface layer (the layer containing cyclic olefin). If stretching at low temperature in order to improve the shrinking properties, there is the problem that shrink finishability worsens, since, in addition to the natural shrinkage becoming too large, decreasing the dimensional stability, the stress at heat-shrinking time becomes also high.
Meanwhile, a heat-shrinkable polyolefin series laminated film resulting from stretching a laminated layer body having a layer containing a polyolefin series resin with a density of less than 0.94 g/cm3, and a layer containing a composition of 70% by mass or more but no greater than 90% by mass of a cyclic olefin series resin and 10% by mass or more but no greater than 30% by mass of a crystalline polyolefin series resin, has been disclosed (see for instance Japanese Patent Application Laid-open No. 2000-202951).
The above film has problems, such as, although the problem of meeting a specific gravity of less than 1.00 (hereinafter referred to as “decreasing the specific gravity”) is addressed, as the surface layer is a mixed resin of cyclic olefin and high density polyethylene, due to the difference in the elasticity ratios of the two constituents, surface roughness of the film occurs at stretching time or shrinking time, decreasing transparency. In addition, there are problems, such as, since the firmness (stiffness at ordinary temperature) of the entirety of the film is low, the yield tends to decrease when using a labeling machine and the like to cover a container such as a PET bottle with a film formed into a bag, due to the bottle being covered skewedly, creasing of the film, and the like.